1. Field of the Invention
The present invention is directed to automatic calibration control of a CRT monitor which allows the light output response of the monitor to be initially adjusted and maintained as the monitor ages and, more particularly, to using a single channel light meter to produce a light output response curve of the monitor, which curve is used with an aim or preferred tone scale curve to create a video shaper, where the video shaper modifies the code values sent to the monitor when a picture, stored as a digital image, is being reproduced.
2. Description of the Related Art
In automated print shops and other color image reproduction facilities, it is critically important that electronic images displayed at different times maintain consistent and accurate color characteristics. For example, when a layout artist is designing a full color layout, for example a magazine, using a computer based system, a digital image displayed on the monitor (CRT) today must have the same color and tone scale reproduction as when the image is displayed in the future. In addition, the displayed image should match as closely as possible the image as it is reproduced in hard copy format, that is, the images should have the same tone scale. However, the color fidelity of CRT monitors degrade over time due to component aging and a stored digital image will appear to change color over time because of this degradation.
A CRT used in color image reproduction facilities should be selected to have good registration and minimum color crosstalk. Such CRT's are generally factory calibrated to have a particular color temperature, for example D.sub.65 (6500.degree. K.), dynamic range (black to white) and grey scale tracking (color constancy from black to white).
During use of an image processing system, including a calibrated monitor, a color image can be digitally stored in many different formats. One storage format is digital log exposure code values, where the log exposure of the image is stored as red, green and blue color values for each pixel in the image. Since these log exposure code values are frequently stored as floating point values, it is necessary to go through a conversion to an integer format, for example, conversion to digital exposure code values for the monitor, which may be used in building/accessing a video shaper. If other formats, such as color difference signals or CIELAB, are used, the conversion to digital exposure code values for the monitor must include transformation of the data back to basic components.
In the color reproduction industry different aim or preferred tone scales are used for different purposes (see Photographic Materials and Processes by Stroebel et al. Focal Press, 1986) and typically a monitor is not adjusted to produce a particular tone scale. As a result, users must estimate how a picture displayed on the monitor will actually be reproduced when made into a hard copy, as well as guess about the changes that have occurred in the monitor over time.
What is needed is an automatic method of calibrating a monitor to a desired tone scale and then controlling the monitor at that same tone scale as the monitor ages, so that the user will see on the CRT display what will actually be produced in the hard copy version.